This invention relates to charge transfer devices and in particular to devices in which charge carriers may be transferred in two noncollinear directions.
"Charge Transfer Device" (CTD) has come to be known as the generic description for devices which store and transfer charge carriers in a storage medium by means of appropriate potentials created at or near the surface of the medium by a series of electrodes disposed upon an insulating layer overlying the surface of the medium. These devices may be of the charge coupled (CCD) or bucket brigade (BBD) type. In the basic bucket brigade device, regions of fixed charge are provided in the storage medium beneath each electrode and extending slightly into the area below an adjacent electrode in the charge transfer path. When an electrode is biased, the region of charge immediately under it is reversed biased and the channel between this region and its neighbor is inverted to permit the transfer of charge. Thus, mobile charge carriers are stored in fixed charge regions as majority carriers and transferred through the channels as minority carriers. The basic charge coupled device stores charge carriers under depletion biased electrodes and transfers the charge carriers by creating a succession of potential wells at the storage medium surface along the transfer path. Charge is therefore stored and transferred in the form of discrete packets of minority carriers in the medium.
Most prior art devices have proposed electrode configurations for moving charge carriers in essentially a linear path, including schemes for moving charge in serpentine fashion and for reversing the transfer to the opposite direction. In many information processing applications, such as time division multiplexing, it is necessary to provide for conversion from serial-to-parallel and parallel-to-serial transfer of charge as well as many other types of logic operations. Thus one can make advantageous use of a device which is capable of transferring charge from any particular storage location in either of two noncollinear directions.
In order to meet this need, a device was proposed which utilized an electrode network in two levels of metallization to define noncollinear transfer paths in the medium. (See U.S. patent application of R. H. Krambeck-C. H. Sequin-R. H. Walden, Ser. No. 342,173, filed Mar. 16, 1973 and assigned to the present assignee.) In some cases, two levels of metallization may present some fabrication difficulties. Hence, it is desirable to provide an alternative structure which needs only one level of metallization in order to transfer charge in two noncollinear directions.